Production of synthetic rubber



March 19, 1946.

l. L. WOLK ET AL PRODUCTION OF SYNTHETIC RUBBER Filed Deo. 14,- 1942ATTORNEYS A improved heat exchange.

rarement. 19, 194s. l

I. Louis Wolk and John D;

v Okla., ors to Phillips Petroleum Com- PRODUCTION or sYNTnETrc armenaUpham Bartlesville,

pany, a corporation of Delaware Application December 14, 1942, SerialNo. 468,958

' 9 claims. (Cl. 26o-sas) thetic rubber latex by freezing by comminglingthe latex intimately with a liquid refrigerant such as propane. In sucha process it is desirable to use heat exchange expedients in order toutilize the refrigeration as emciently asposl sible. Ordinarily indirectheat exchange methods would be used, and if perfect exchange werepossible, and no heat entered the cold parts of the system fromextraneous sources, the net refrigeration required would be only thatnecessary to cool the latex from the polymerizing temperature at whichit is available down to room temperature. Of course, such perfectexchange is not attainable in actual practice, but we have discovered amethod of accomplishing coagulation of the latex by freezing in a veryeconomical manner by an yimproved method involving heat exchange.

The principal object of the present linvention is to .provide animproved process of ooagulating synthetic rubber latex by freezing.Another object is to provide such a method employing an Another objectis to provide such a method wherein a direct heat exchange step is usedto effect a substantial part of the total heat exchange employed in thestep of coagulatng by freezing, the balance being advantageously e'ectedby indirect heat exchange. Numerous other objects will hereinafterappear.

The accompanying drawing portrays diagrammatically one arrangement ofapparatus that may be advantageously employed in carrying out thepresent invention in one embodiment. While the modification illustratedis essentially a continuous process, the principles of the invention maybe readily adapted to batchwise operation by those skilled in the art.

In accordance with the present invention the synthetic rubber latex, i.e., the emulsion of polymer derived from the polymerizer, is frozen inany suitable manner. The frozen latexis placed in 'a suitable vessel orzone andintimately andu mediate cooling step. The warmunfrozen latexbelow room temperature.

and the frozenf latex are intimately admixed with each other in order tobring about thermal equilibrium.

Preferably the temperature of the liquid latex v so admixed with thefrozen latex, the temperature of the frozen latex, and the relativeproportions of unfrozen and frozen latex admixed in the mixing zone areso adjusted prior to entry into the zone that, after thermal equilibriumhasl been established, the resulting mixture is at the freezing pointand comprises substantial proportions of each of the frozen and meltedphases.

In a continuous operation such as that portrayed in the `drawing, it isespecially preferred to feed equal proportions by weight of the frozenand unfrozen latex to the mixing zone and to so adjust the temperaturesof each incoming stream that when thermal equilibrium between the`frozen and unfrozen phases is established, `onehalf of the totalmixture by weight is in the liquid state and one-half is in the solidstate. In such an operation the liquid phase is continuously withdrawnand frozen in a separate zone to supply the frozen latex fed to themixing zone.

When such thermal equilibrium has been established, the liquid phasecomprises predominantly unfrozen latex along with minor amounts of theaqueous phase from thawed latex, while the solid phase will comprisepredominantly frozen latex with small quantities thawed latex.

It is preferred that al1 parts of the apparatus be well insulatedagainst heat because they are for the most part at temperaturessubstantially This lowers the refrigeration requirements and promoteshigh thermal emciency. It is especially important that the mixing zonewherein the unfrozen and the frozen latex streams are admixed be wellinsulated so that substantially adiabatic conditions prevail therein.Ordinarily no additional cooling by heat exchange with an extraneouscoolant takes place in the mixing zone, although such added cooling maybe employed ifl found desirable.

After mixlnghas been accomplished and thermal equilibrium established,the solids are separated from the liquid ph'ase. The liquid phase whichis usually at substantially the freezing temperature is passed to arefrigeration unit where it is frozen to provide the frozen latexintroduced to the mixing zone.

The separated solid phase isv heated to thaw it, and the precipitated ywashed and passed to further utilization steps.

of rubber precipitated from rubber is then separated;

. mixing zone.

i As a further and advantageous feature of our invention, the thawing ofthe solid frozen latex separated from the mixture resulting from themixing step may be at least partially accomplished by passingliquidlatex direct from a polymerizer in indirect heat exchange with thesolids to be thawed. This makes possible the most effective utilizationof the refrigeration, and the warm latex is brought to the propertemperature forits next step of mixing with frozen latex in the mixingor direct heat exchange step described above.` Cooling the warm latex inthis manner does notcomplete the thawing of the frozen latex and thepartially thawed latex may finally bepassed in indirect heat exchangewith warm, compressed refrigerant used in the freezing'step, to causefurther heating or thawing of the frozen latex. Alternatively or inaddition, the partially thawed latex may be passed in heat exchange with1media. used for cooling the polymerizers. In any event a final thawingstep may be employed. After thawing, the coagulated rubber is separatedfrom the aqueous phase which may conveniently be recycled to theemulsion forming step wherein the monomer is emulsifled prior topassageto the polymerizer.

An important advantage is the rapidity of the heat transfer by directheat exchange in the It was not to be foreseen that the frozenA latexand the unfrozen latex could be directly admixed with one another withsatisfactoryresults, since in ordinary processes it is wholly impossibleor unfeasible to mix a product stream directly with an incoming streamto secure direct heat exchange. The overall refrigeration requirementsare as low as possible since the heat of fusion of the latex is utilizedcompletely. The direct heat exchange step achieves an unusually rapidchange in temperature for outgoing frozen latex and incoming liquidlatex and reduces the amount of indirect heat exchange required, and inmany cases eliminates need for indirect heat exchange. As will be wellunderstood by those skilled in the art,- indirect heat exchange is veryunsatisfactory since it requires elaborate equipment, the heat transferis imperfect', and it is impossible to I obtain therapid and perfectthermal equilibrium obtained in the direct heat exchange step of thepresent invention.

The cycle as thus described is continued indefinitely either batchwiseor continuously, new

` liquid latex being admixed with freshly frozen latex, the solidsseparated from the liquids, etc.

Heat exchange requirements of prime imporl tance in the thawing of thefrozen latex are met by the process described herein. One is thatgreatest possible use is made of the refrigerating capacity of thefrozen latex whereby the overall refrigeration requirements are kept ata minimum. Another is `that heat exchange be as enicient as possible sothat in a batch process the time required-for the various operations bekept facilitates rapid and intimate admixture with the liquid latex.` Asthe refrigerant, liquefied petroleum gas, especially liquid propane, ispreferred.

Under exceptional circumstances, though ordinarily less preferably,operation may be so conducted that all of the relatively warm orunfrozen latex is frozen in themixing zone by the. frozen latex, thelatter being at'a temperature sufficiently below thefreezing point -andbeing introduced in quantity suiilcient to effect freezing of the entireamount of the warm latex.

At the other extreme, and likewise less preferably, the mixing step isso conducted that all of the frozen latex is melted therein by theunfrozen latex, whereby the solid phase consists only of coagulatedrubber. Such operation is normally undesirable because of the dilutingeffect of the aqueous phase derived by the thawing of the frozen latex.

Referring now to the accompanying drawing, frozen latex, say at 20 C.,is fed via line I into mixing zone 3 wherein it is admixed withrelatively warm unfrozen latex, say at 23.75 C., fed via line 2. Anyresidual refrigerant may leave vessel 3 in vapor form via line 4. Afterattaining equilibrium, the resulting mixture of solid and liquid whichis at the freezing point, say 2 C., is.

and. liquid phases takes place. 'I'he cooled latex which is at thefreezing point, say 2 C., is

passed via. line 6 to the freezing zone 1 where liquid propaneintroduced via line 8 is intimately contacted withit to freeze it, thefrozen latex being removed from freezer I by line I equipped with ascrew conveyor. A

The solid phase separated in zone 5 is removed via line 9 also equippedwith a conveyor screw. This solid phase is also at the freezing point,say 2 C. and comprises frozen latex as well as any solid rubbercoagulated by any thawing of frozen latex in the mixing step.

The solid phase in line 9 passes in indirect heat exchange withrelatively warm (say at 30 C.) fresh latex enteringthe system via lineI0. This is done in heat'exchanger Il, the latex thereby cooled to say335 C. leaving via line 2 and thenceA direct) heat exchange with warmpropane passing from refrigerant compressor via line I4 and returning toan expander and cooler I5 via line I6. The cold liquid propane leaves I5via line 8 and passes into freezer 1. The propane gas formed in I5leaves via. line I1, joins propane gas leaving short or in a continuousprocess the equipment be of small size and simple. In accordance withour invention, direct heat exchange is at a maximum while indirect heatexchange between solids and -liquids which is especially diilicult is ata minimum.

The process is particularly applicable when the freezing step isconducted in accordance with the copending application of Wolkabove-identified wherein freezing is accomplished by intimately anddirectly admixing the liquidlatex with a refrigerating gas in liquidstate and allowing evapofreezer 'I via line I8 for passage to compressorI3.

The thawed latex passes from thawing zone I2 to separation vessel I9where the aqueous phase is removed via line 20 and therubber coagulumvia line 2I equipped with conveyor screw. The aqueous phase isadvantageously recycled to that portion of the rubber plant where theemulsion of monomeric unsaturated compounds is prepared.

Although in the drawing the solid phases in separators 5 and I9 areshown as being below the liquid phase, this will not always be the case.Whether the solids float or sink will depend upon the particular type ofsynthetic rubber latex being treated. The densities of differentlatices, both other known methods, as well as by the gravita-- tionalsettling shown.

The freezing point of the latex will depend upon its composition,particularly as to the concentration of electrolytes and polymertherein. Furthermore, although ideally the final temperature in mixingzone 3 and separator 5 will be equal to the freezing point of the latexbeing treated, it may in some cases be somewhat higher or lower,especially if it is more economical over all not to'wait for attainmentof complete thermal equilibrium therein. In such case, thetemperaturesof the separated latex and solid phases may still be a few degreesapart.

The latexes preferably treated in accordance with our invention are theproducts of polymerizationin aqueous emulsion of an aliphaticconjugatedsdiolen such as butadiene, isoprene, or substituted orunsubstituted homologs thereof, polymerized either byitself ory inconjunction with vone or more other unsaturated materials, especiallyvinyl compounds such as acrylonitrile or styrene. Such polymerizationmay be accomplished in known manner, for example as described in' U.S..Patents 1,938,731 and 1,973,000', or by newer methods ormodifications known to the-art. The invention may be applied also toother emulsions such as natural rubber latex or the like, but isordinarily most advantageous in cases wherein it is important to readilyrecover 'emulsifying agents and unreacted material as in the case ofsynthetic rubber emulsions.

We claim:

mixture.

zone and wherein substantially equal proportions by weight of said solidland liquid phases are present in and removed from said resulting 4.'I'he process of coagulatlng a synthetic rubber emulsion produced by theaqueous emulsion polymerization of an aliphatic conjugated dioleiin,which comprises freezing one portion of said emulsion, admixing theresulting frozen emulsion with another portion of said emulsion inunfrozen condition in such proportions that the unfrozen emulsion iscooled without substantial thawing of the frozen emulsion, 'separatingthe l frozen emulsion from the unfrozen emulsion,

freezing the unfrozen emulsion, thawing the frozen emulsion, andseparating coagulated synthetic rubber therefrom.

5. A process according to claim 4 wherein the synthetic rubber emulsionis produced by the emulsion polymerization of an aliphatic conjugateddiolen and an unsaturated material copolymerizable therewith.

`6. A process according to claim 4 wherein the synthetic rubber emulsionis produced by the emulsion polymerization of a conjugated dioleiin andan unsaturated material copolymerizable therewith containing a vinylgroup.

1. 'The process of coagulating a synthetic ruby ber latex consisting ofan aqueous dispersion of a copolymer of butadiene and styrene, which fcomprises freezing said synthetic rubber latex in a freezing zone,admixing the resulting frozen latex with a portion of said syntheticrubber latex in unfrozen condition in a mixing zone, allowing theIestablishment of thermal equilibrium in the resulting mixture, soadjusting the temperatures and relative proportions of the frozen andunfrozen latex so admixed that said resulting mixture is at the freezingpoint and contains substantial proportions of both solid and liquidphases, separating the solid lphase from the liquid phase, and freezingthe liquid phase in said freezing step.

2; The process of claim 1 wherein said solid phase is passed in indirectheat exchange relationship with saidV portion of unfrozen syntheticrubber latex whereby the latter is cooled on its way to said mixing zoneand heat is simultaneously imparted to said solid phase, and whereinsaid solid phase is thawed to effect separation of the rubber contentthereof.

3. The process of claim 1 carried outcontinuously and whereinsubstantially equal proportions by weight of said unfrozen and frozensyn- 7. AA process according to claim 4 wherein thesynthetic rubberemulsion is produced by the emulsion polymerization of butadiene andstyrene. r

- 8. The process of coagulating a synthetic rubber emulsion produced bythe 'aqueous emulsion polymerization of a conjugated diolefln whichcomprises freezing a portion of said v'emulsion in a freezing zone,admixing the resultant frozen emulsion with a portion of said emulsionin unfrozen condition in a mixing zone, allowing the establishment ofthermal equilibrium in the resulting mixture, so adjusting thetemperature and relative proportions of the frozen and unfrozen emulsionso admixed that said resulting mixture is substantially at the freezingpoint and contains substantial proportions of both solid and liquidphases, separtaing the solid phase from the liquid phase, and freezingthe liquid phase in said freezing step.

9. The process of coagulating a synthetic rubber emulsion produced bythe aqueous emulsion polymerization of a conjugated dioleiin whichcomprises intimately comingling a portion of said emulsionand aliqueed'normally gaseous refrigerant in a freezing zone and effectingfreezing of said emulsion by evaporation ofsaid refrigerant, intimately.admixing the resultant frozen emulsion with a portion of said emulsionin unfrozen condition in a mixing zone, allowing the Y establishment ofthermal equilibrium in the rethetic rubber latex are introduced to saidmixing sulting mixture, so adjusting the temperature and relativeproportions of the frozen and unfrozen emulsion so admixed that theresulting mixture is substantially at the freezing point and containssubstantial proportions of both solid and liquid phases, separating thesolid'phase from the liquid phase. freezing the liquid phase in saidfreezing step, and-thawing said solid phase to effect separation of therubber contained therein.

I. LOUIS WOLK. JOHN D. UPHAM.

